Method for producing ultrafine grained steel

ABSTRACT

A METHOD OF PRODUCING ULTRAFINE GRAIN SIZE IN STEEL COMPRISING CYCLICALLY TREATING SUCH STEEL BY HEATING IT AT TEMPERATURE AT WHICH AUSTENITE GRAINS COARSEN RAPIDLY FOR JUST SUFFICIENT TIME TO TRANSFORM IT TO AN AUSTENITIC STRUCTURE AND THEN BEFORE ANY APPREICALB E GRAIN GROWTH OCCURS QUICKLY COOLING IT TO TRANSFORM THE ASUTENITE IN ALL BUT THE LAST CYCLE TO A MICROSTRUCTURE OF THE CLASS CONSISTING OF MARTENSITE, BAINITE OR MIXTURES AND IN THE LAST CYCLE COOLING AT A RATE WHICH WILL PRODUCE THE DESIRED MICROSTRUCTURE WHEREBY AN AUSTENITE GRAIN SIZE FINER THAN ASTM #10 IS PRODUCED THEREIN.

Oct. 10, 1972 R. A. GRANGE METHOD FOR PRODUCING ULTRAFINE GRAINED STEELOriginal Filed June 3, 1965 CYCLE 2 Nd. CYCLE Sac.

TEMPERATURE,

ouewcw //v LEAD AT /500/-' ROOM TEMR INITIAL STRUCTURE (HOT ROLLED)OUEA/CH l/V LEAD 47' I500 "F.

TIME Z Nd.

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CYCLE CYCLE OUENCH Ml LEAD AT I500F.

OUEIVCH IN LEAD 47' I500 "F 4 Th. CYCLE CYCLE ASTM ASTM

No. I!

IN VE/V TORS.

RA Yam/v0 A. GRANGE and EDWARD R. .SHA cxarono Attorney United StatesPatent Oflice Matter enclosed in heavy brackets [1 appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE A method of producing ultrafine grain size insteel comprising cyclically treating such steel by heating it at leasttwice to above its A temperature but below the temperature at whichaustenite grains coarsen rapidly for just sufficient time to transformit to an austenitic structure and then before any appreciable graingrowth occurs quickly cooling it to transform the austenite in all butthe last cycle to a microstructure of the class consisting ofmartensite, bainite or mixtures and in the last cycle cooling at a ratewhich will produce the desired microstruc-' ture whereby an austenitegrain size finer than ASTM is produced therein.

This invention relates to the production of ultrafine grained steel andmore particularly to the production of ultrafine grain size in steel byheat treatment.

The desirability of fine grain size in steel has long been recognizedbecause of its elfect on the strength, ductility and other mechanicalproperties of steel. Heretofore fine grain has only been obtainable bysteel production techniques such as by deoxidizing with silicon andaluminum or the addition of grain refining elements such as vanadium orcolumbium. Steels containing aluminum, vanadium, columbium and othergrain refining elements in sufiicient amounts to refine the grains arefine grained and resist grain coarsening in that they can be heated wellabove the Ac temperature for considerable.

time. However, the use of such elements adds to the expense of thesteels and particularly in the case of aluminum results in a killedsteel so that hot topping practice is required. Moreover the obtainingof fine grain in such manner precludes the obtaining of a relativelypure rim portion such as results from a full rimming action. Applicantshave discovered that it is possible to produce even finer grains than isconventionally obtained with deoxidizing or grain refining elements bysuitable heat treatment alone. The fine grain so produced is finer than#10 on the ASTM grain size chart and may accordingly be termed ultrafinegrained.

Thus it is an object of the present invention to obviate the necessityof using deoxidizing or grain refining elements in the production offine grained steels.

Another object is to produce an ultrafine grained condition in steel byheat treatment following forging or rolling the steel to the desiredshape or gauge.

The foregoing and further objects will be apparent from the followingspecification when read in conjunction with the attached drawingillustrating the teaching of the invention as applied to SAE 1045 steel.

We have discovered that an ultrafine grain size can be produced in steelby a multicycle heat treatment consisting of rapidly heating the steelin less than seconds to a temperature above the [Ac AC3 to convertferrite to austenite but below the temperature at which austenite grainscoarsen rapidly and as soon thereafter as possible cooling it before theaustenite grains soformed can grow appreciably. The cooling in all butthe last cycle must be sufliciently fast to develop a structure of theclass consisting of bainite, martensitc or mixtures thereof and thusavoid transformation to a ferrite-pearlite microstructure. In the lastcycle, air cooling, which develops a ferrite-pearlite microstructure,may be used if desired. Preferably the steel is not heated more than F.above its [Ac Ac temperature. In some steels two such cycles willproduce substantially the finest grains possible although in otherscontinued refinement may result by repeating the cycle three or moretimes. The optimum number of cycles varies among steels of differentcompositions depending primarily upon the nature of the initial andintermediate metallographic structures developed by cooling after eachcycle.

The steels suitable for grain refinement by this process are all thosewhich are hardenable by heat treatment, are predominantly ferriticrather than austenitic at room temperature, and become completelyaustenitic on heating to a suitable elevated temperature. This excludesonly highalloy steels such as austenitic or ferritic stainless steels.At room temperature, steels responsive to our process consist ofmartensite or of carbides in a matrix of ferrite. When heated into thecritical range, ferrite begins to transform into austenite by anucleation and growth process. Ferrite grain boundaries are preferrednucleation sites for austenite, and furthermore there is a strongtendency for austenite once nucleated to grow until it has consumed theparticular ferrite grain in which it nucleated and then to cease grom'ngfor a time. Thus, as steel is heated into its critical range, austenitegrains assume the shape and size of the prior ferrite grains. However,this is only a temporary situation because growth of the austenitegrains is also occurring, by which process more stable grains consumeadjacent less stable grains. If the heating rate is relatively fast, theaustenite grain growth process is slower than the ferrite to austenitetransformation. This makes it possible to retain in the final product anaustenite grain size closely related to the prior ferrite grain size byrapidly heating to a temperature just sulficiently above the criticalrange to austenitize the steel and then cooling immediately. Theresulting ferrite grain size is smaller than the parent austenite grainsize. Thus, heating rapidly so as to expose austenite to temperaturesabove the critical range for a time barely sufficient to convert allferrite to austenite and cooling quickly results in refinement of grainsize. Furthermore, repeating this short heating cycle producesadditional grain refinement because at the start of the second cycle theferrite grain size is smaller than at the start of the first ReissuedOct. 10, 1972 cycle. Thus, mulicycle rapid heating leads to progressivegrain refinement. After a certain number of cycles, depending on theparticular steel and specific heating cycle, no additional grainrefinement is produced, however, because growth occurs more and morerapidly as austenite grains are made ever finer on successive heatingcycles until eventually a stalemate is reached.

It requires at least two cycles to obtain the maximum grain refinementand about four cycles will almost always develop the maximum refinement.The number of cycles to produce such result varies depending primarilyupon steel composition and upon the nature of the initial andintermediate metallographic structures developed by cooling after eachcycle.

In practicing our invention, it is necessary that the heating be donequite rapidly but once the desired rate is obtained on further advantageresults from exceeding such rate by extremely fast heating. Inthicknesses up to 0.5 inch, satisfactory results can be obtained byleadbath heating but other types of liquid baths, such as salts, orelectrical induction or resistance heating may be used. The heating timeshould be less than 60 seconds and preferably less than 20 seconds. Insuch thicknesses as .03 to .50 inch, the same ultrafine grain size wasobtained upon heating in a lead bath from to seconds.

While the treatment may involve cooling to room tem perature afteraustenitizing, it is only essential to cool sufficiently to insuretransformation of the austenite to bainite, martensite or mixturesthereof and avoid the formation of ferrite-pearlite on all cycles exceptpossibly the last. The cooling may be done in any convenient manner butmust be sufiiciently rapid to minimize austenite grain growth attemperatures above the critical range. Thus for thin sections airquenching will suflice but heavier sections will require water or otherliquid quenching. After the final heating cycle, the steel should becooled at a rate which develops the desired metallographic structure forthe intended use of the product. No additional heat treatment other thansubcritical tempering or annealing should be used thereafter because theultrafine grain achieved by the treatment may be wholly or partiallylost.

The single figure of the drawing illustrates schematically a four-cycletreatment showing the degree of grain refinement obtained by one, twoand four cycles in comparison with the hot rolled structure. The steelused in obtaining the data for the figure was SAE 1045 which had beenhot rolled to 0.150-inch-thick strip. Heating was accomplished byimmersing in a lead bath maintained at 1500- F. for 20 seconds.

The [Ac A0 temperature of this steel is 1420 F. As shown, an ultrafinegrain size of #11 ASTM was obtained in two cycles, with slightly finergrain resulting from two additional cycles. No appreciable improvementwas obtained by additional cycles beyond the four shown. In the cyclictreatment of FIGURE '1, the specimens were oil quenched after the firstcycle of the two-cycle specimen and after the first, second and thirdcycle of the four-cycle specimen. The final cooling in each case was aircooling.

The following Table I gives data obtained from treating representativehypoeutectoid steels in accordance with the invention. This tablecompares the grain size obtained by one, two, and four cycles with thatobtained by the indicated conventional austenitizing treatment. Themedium carbon steels often developed the full improvement in grain sizewith two cycles but the low carbon steels required more than two cyclesto do so. The difiiculty in developing full grain refinement in lowcarbon steels is believed to result from their higher [Ac A0 temperaturewhereby they must be heated to higher temperatures to austeuitizc them.

TABLE I Grain refinement in hypoeutectoid steels Austenlte grain size,ASTM No.

Rapid heating 1 Conventional 1 Grade SAE No. austenltizlng 1 cycle 2cycles 4 cycles 4 l Heated in air for 20 minutes at 1,550 F. (except1,600" F. for 1015 and 43 i LE feated in lead for 10 seconds at 1,550"F. (except 1,600" F. for 1015 and tI;l'0rE.-All steels heat treated inform of hot-rolled 0.100-lnch-thiek s r p.

The following Table II gives data obtained from treating high carbonsteels pursuant to the invention. If such steels are heated sufiicientlyhigh to dissolve all the carbides, very little grain refinement can beobtained. However, a high degree of grain refinement can be obtained ifthe heating temperature is too low to dissolve all the carbides. Sincefor many uses solution of all carbides in high carbon steels is notrequired or desirable such steels can in many cases be advantageouslytreated to produce ultrafine grain size.

1 Heated for 10 seconds in lead.

Norm-Specimens were 0.100-inch-thick strip.

As indicated hereinabove the optimum heating temperature and time mayvary with steel composition but generally the optimum temperature willbe about the same as that conventionally used for austenitizing eachsteel. While the maximum grain refinement is generally obtained in twoto four cycles, further cycles may in some cases be requiredparticularly with hypereutectoid steels wherein the dispersion of smallundissolved carbide particles developed minimizes grain growth.

The following Table III shows that the type of deoxidation has little,if any, efiect on the grain size obtained by practicing the invention.Thus in the case of 0.1- inch-thick specimens of 1045 steels tested,steel produced by coarse-grain practice, i.e., silicon-killed had thesame fine grain size #14 ASTM after four cycles as a steel produced tofine grain practice, i.e., killed with silicon and aluminum.

l McQuaid-Ehn Test.

Due to the ultrafine grain size, steels treated in accordance with thisinvention have better toughness, ductility TABLE IV Mechanicalproperties [AISI 1045-0.23-inch thick strip] becomes ferritic on coolingto room temperature comprising cyclically treating such steel by heatingit at least Notch l Tensile Yield Elong. Reduction toughness Asutenitestrength, strength, in 1", of area, ./1b

Heat treatment grain size p.s.i. p.s.i. percent percent at 25 F.

Conventional Normalize #8 ASTM. 101, 900 64, 100 28 57 11 (1,550 F. aircool).

Cyclical Rapid Heating #12 ASTMMU 99, 200 73, 200 30. 5 61. 5 20 1V-Notch Charpy Impact Test; average of three half-width specimens. Whilewe have shown and described several specific embodiments of ourinvention, it will be understood that these embodiments are merely forthe purpose of illustration and descrciption and that various otherforms may be devised Within the scope of our invention, as defined inthe appended claims.

We claim:

1. A method of producing ultrafine grain size in steel which ispredominately ferritic rather than austenitic at room temperature,becomes completely austenitic on heating to suitable elevatedtemperature and thereafter becomes ferritic on cooling to roomtemperature comprising cyclically treating such steel by heating it atleast twice to above its [Ac A0 temperature but below the temperature atwhich austenite grains coarsen rapidly for 3 just sutficient time totransform it to an austenitic structure and then before any appreciablegrain growth occurs quickly cooling it to transform the austenite in allbut the last cycle to a microstructure of the class consisting ofmartensite, bainite or mixtures thereof and in the last 3 cycle coolingat a rate which will produce the desired microstructure whereby anaustenite grain size finer than A'ST-M #10 is produced therein.

2. A method of producing ultrafine grain size in steel which ispredominately ferritic rather than austenitic at room temperature,becomes completely austenitic on heating to suitable elevatedtemperature and thereafter References Cited The following references,cited by the Examiner, are of record in the patented file of this patentor the original patent.

UNITED STATES PATENTS 7/1933 Morrill 148-144 OTHER REFERENCES Principlesof Heat Treatment, by M. A. Grossmann, published by the A. S.M. (1935),p. 203 relied upon.

RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

